1. The Field of the Invention
The present invention is directed to a method of standardizing a basis weight gauge and more particularly to a method in which the gauge scans sheet material.
2. Description of the Prior Art
Due to the extreme ambient conditions under which basis weight gauges must operate there is an inherent tendency of the parameters of the gauge to drift. A portion of this drift is due to temperature changes and another portion to dirt buildup in the radiation path of the gauge. Thus, the gauge indication of basis weight must be compensated or standardized.
One method of standardization is disclosed in U.S. Pat. No. 3,681,595 in the name of Dahlin and assigned to Measurex Corporation. The patent teaches a method of standardizing a basis weight gauge for measuring the weight per unit area of a sheet material. The gauge has a radiation source and radiation detector with a gap therebetween for directing radiation along a path toward the material and for detecting the amount of radiation transmitted through the material. The transmitted radiation is a measure of the basis weight of the material substantially in accordance with Beer's law. The gauge as taught in the patent also includes standard means having a predetermined and stable basis weight and movable to be interposed in the radiation path.
The method includes the following steps: (a) providing at least one reference sample of a material having a measured basis weight; (b) placing the reference sample in the gap and obtaining a measurement of the radiation transmitted through the sample; (c) constructing a first calibration curve from the measurement of the material of known basis weight, the curve relating basis weight as a function of transmitted radiation substantially in accordance with Beer's law; (d) constructing a second calibration curve by the simulation of dirt in the radiation path whereby the second curve is displaced from the first curve; (e) placing the standard in the gap and measuring an initial shift due to the dirt simulation; (f) obtaining a function of the calibration curve displacement as a function of one of the two variables of the calibration curves; (g) updating the displacement function for the effect of current ambient conditions including the effects of air density changes due to temperature variation and other temperature effects and dirt buildup in the radiation path including the steps of
(1) scanning offsheet and obtaining a current value of transmitted radiation with no sheet material in the radiation path but with the standard in the path to obtain a current standard shift, PA1 (2) updating the displacement function by relating the current standard shift to the initial standard shift; and
(h) scanning the sheet material and measuring the current transmitted radiation and relating the updated displacement function to the first calibration curve along with such current measurement to obtain a standardized basis weight value.
The method of standardizing according to the patent is suitable for many purposes, but it has certain drawbacks.
For example, in some materials such as bi-axially oriented plastic film the basis weight varies significantly across the sheet. In such a case the system described in the patent may not be completely effective.